IS 1892:1979 is the Indian Standard (BIS) for site investigations for foundations. This code establishes standard procedures for conducting subsurface investigations for foundation design. It provides comprehensive guidelines on the methods of exploration, sampling techniques, groundwater monitoring, and specifies the recommended depth and spacing of boreholes based on the structure type.
Provides guidelines for planning and carrying out site investigations for foundations, including boring, sampling, and in-situ tests.
Code of practice for site investigations for foundations. Borehole spacing, depth, sampling, testing scope.
| Reference | Value | Clause |
|---|---|---|
| Borehole spacing — large building | 10–30 m c/c | Cl. 5.2.1 |
| Borehole spacing — small building | as project requires | Cl. 5.2.1 |
| Borehole spacing — long structures (highways) | 150–300 m c/c | Cl. 5.2.1 |
| Borehole depth — minimum (light loads) | 1.5 × foundation width below founding | Cl. 5.3 |
| Borehole depth — heavy structure | to firm strata or 2 × footing width | Cl. 5.3 |
| Borehole depth — deep foundations | min 5 m below pile toe | Cl. 5.3 |
| Number of boreholes — minimum | 3 (for any foundation design) | Cl. 5.2.1 |
| SPT sampling interval | every 1.5 m or change of strata | Cl. 5.4.2 (refers IS 2131) |
| Undisturbed sample interval — clay | every 3 m or per layer | Cl. 5.4.4 |
| Disturbed sample interval | every change of strata | Cl. 5.4.3 |
| Standard penetration sampler — N value | blows for last 300 mm of 450 mm drive | (IS 2131) |
| Lab tests — minimum (per soil type) | grain size + Atterberg + density | Cl. 6.1 |
| Shear strength tests — clay | UCS, triaxial UU/CU per IS 2720 | Cl. 6.2 |
| Consolidation test — clay layers | 1 per stratum > 1 m thick | Cl. 6.2 |
| Groundwater observation period | monitor for ≥ 1 monsoon cycle if critical | Cl. 5.6 |
| Plate load test — important structures | at least 1 per soil type | Cl. 6.4 (refers IS 1888) |
| Geophysical surveys — supplement | seismic refraction / resistivity | Cl. 4.5 |
| Investigation stages | preliminary → detailed → confirmatory | Cl. 4.1 |
| Report contents — minimum | soil profile, strength, GW, recommendations | Cl. 8 |
| Backfilling of boreholes | with bentonite-cement grout | Cl. 5.7 |
| Parameter | IS Value | International | Source |
|---|---|---|---|
| Borehole Spacing for Buildings | For a compact building site (<0.4 ha), one at center and each corner. For larger areas, a grid of 30-50m is suggested. | Judgement-based; spacing should detect critical geological variations. A grid of 15-25m may be appropriate for high-rise or industrial structures. | BS 5930:2015 |
| Depth of Exploration for Spread Footings | A depth equal to 1.5 times the width of the footing below the foundation level. | Depth where the net stress increase is a small fraction (e.g., <20%) of the initial effective overburden stress, or where strata are proven competent. | EN 1997-2:2007 |
| Standard Penetration Test (SPT) Hammer | 63.5 kg hammer with a 75 cm free fall (as per IS 2131). | 63.5 kg hammer with a 760 mm (76 cm) free fall. Modern practice requires reporting the energy ratio (ER) and correcting N-values to a reference ER (e.g., N60). | EN ISO 22476-3:2005 |
| Undisturbed Sample (UDS) Quality | Area Ratio should not exceed 20% in any case; preferably less than 10% for sensitive soils. | Sample quality is classified (e.g., Class 1-5). High-quality samples (Class 1) require methods where Area Ratio is typically <15%. | EN ISO 22475-1:2021 |
| Trial Pit Maximum Depth | Generally up to 3m; can be extended to 6m with special shoring and precautions. | Practical limit is typically 4.5m for safety and excavator reach. Deeper pits require extensive engineered support systems. | BS 5930:2015 |